wintering koi

Could someone please explain the propper way to keep koi in my garage over winter? I keep my garage around 40 all winter. i have a extra 150 gallon tank. DO i need a filter or just a air stone at that temp. with water this temp do i still need to test the water. i have never wintered fish like this and i am afraid to kill them. does the benifical bacteria still work at that temp i was told that it doesn't?

Could someone please explain the propper way to keep koi in my garage over winter? I keep my garage around 40 all winter. i have a extra 150 gallon tank. DO i need a filter or just a air stone at that temp.
YES to a filter

with water this temp do i still need to test the water.
YES, and do small weekly water changes 15-20%

i have never wintered fish like this and i am afraid to kill them. does the benifical bacteria still work at that temp i was told that it doesn't?
YES, but like the koi it slows down also, stop feeding the fish also below 50`-55`

Grant has a nice winter indoor set up perhaps he will post a link for you